The spring constant or reaction modulus is an important parameter for the engineering considerations involved with materials such as pavement. In pavement engineering, for example, highway and airport runway pavement slabs rest on an elastic foundation. Typically, pavement rests on a base, one or more sub-bases and the natural ground. A defect that arises in one of these layers manifests itself in an alteration of the spring constant of the pavement long before any surface pavement damage can be spotted. Thus, a determination of this parameter for an existing pavement portion is essential for the analysis and design of future top asphalt/concrete layers and the determination of the necessity of such repair work.
Time consuming and expensive plate load tests are currently used for the determination of the modulus of subgrade reaction. This plate load test is commonly used, but the parameters obtained therefrom are merely approximations and the procedure damages the pavement surface tested. As a result, there exists a need for a non-destructive method and, therefore, an apparatus that can be used to determine the spring constant or reaction modulus rapidly and accurately without damage to the pavement surface. Other methods for determining pavement deflection use lever systems or operate on a vibratory force principle.